The industrial production of alkenyl acetates through reaction of olefins, oxygen and acetic acid in vapor phase has been performed in the presence of a heterogeneous catalyst, which comprises noble metals, metals as catalysis promoter in combination with an alkali or alkaline earth metal compound supported on the outer surface of a carrier. This preparation process has been known for a long time and the key point of this process, the evenly distribution of metal components on the surface of a supporting catalyst to provide high yield, also has been widely discussed and studied, mostly based on changing the kind or relative composition of the metal components. However, no matter how the metal composition or its structure has changed, the basic structure of all catalysts for producing alkenyl acetates is essentially a spherical shape catalyst prepared by impregnating on a carrier with the palladium metal, a metal for promoting catalysis, and an alkali or alkaline earth metal compound, wherein the metal for promoting catalysis is preferably gold and copper, and wherein the alkali or alkaline earth metal compound is preferably a potassium compound (U.S. Pat. No. 3,939,199, EP 0361484 and U.S. Pat. No. 4,668,819).
Earlier, the catalyst employed to produce vinyl acetate was prepared by impregnating palladium, gold and the like noble metals on the whole carrier (U.S. Pat. Nos. 3,725,680, 3,743,607); the catalyst employed to produce allyl acetate was prepared by impregnating palladium, copper and the like noble metals on the whole carrier (U.S. Pat. No. 3,917,676). That is, the active noble metals were supported on the interiors and outer surface of a carrier. However, while the catalyst was prepared in this way, it was found in actual reaction that the reactants were scarcely able to efficiently diffuse into inner regions of the carrier, so that the interior active palladium and gold or copper metals were not able to be sufficiently utilized. In order to improve this drawback, in the preparation process of the catalyst for producing alkenyl acetates, usually, instead of impregnating palladium metal and a metal for promoting catalysis on the whole carrier, but to evenly impregnate on the surface of a carrier. That is, to impregnate palladium metal and the metal for promoting catalysis evenly on the surface layer of a carrier to form a spherical shape supported catalyst (U.S. Pat. No. 4,087,622). The preparation method was that the carrier was impregnated with the active metallic materials first, then these metal salts were precipitated by immersing in a solution containing an alkali or alkaline earth metal salts (U.S. Pat. No. 4,048,096, U.S. Pat. No. 3,775,342).
Further, it was known that when preparing the catalyst with palladium and other metals as catalyst promoter in combination with an alkali or alkaline earth metal compound, a special carrier was used. This carrier was washed with acids before the impregnation and treated with bases after the impregnation (EP-A-0519435).
Since palladium and the metal for promoting catalysis were non-homogeneous during the impregnation, which would result in the non-homogenous distribution on the surface of a carrier, therefore, many researches have been made on this problem (U.S. Pat. No. 4,087,622, U.S. Pat. No. 3,822,308 and British Patent 1521652). As a result, in the preparation of the catalyst for producing alkenyl acetates, the catalyst was essentially prepared by impregnating palladium and a metal for promoting catalysis, in combination with an alkali or alkaline earth metal compound on the surface of a spherical shape carrier. The preparation process generally was comprised of the following steps: (1) a carrier was impregnated with an aqueous solution of soluble palladium ions and metal ions for promoting catalysis; (2) the impregnated carrier was immersed in an alkali solution, so that the soluble palladium ions and metal ions for promoting catalysis were precipitated on the surface layer of the carrier and formed into insoluble oxidative state palladium and metal for promoting catalysis; (3) the treated carrier was washed with water to remove soluble ions produced during the precipitation; (4) the oxidative state palladium and metal for promoting catalysis supported on the treated carrier were then reduced to the metallic state; (5) the reduced carrier in (4) was impregnated with a solution of an alkali or alkaline earth metal compound; and (6) the impregnated carrier in (5) was dried. Among these steps, conventionally, the reducing step (4) was performed by a reducing process using liquid reducing agents in liquid phase. However, since non-homogenous nature of palladium and the metal as catalysis promoter on the surface of the carrier, the catalyst prepared by this method is usually unsatisfactory.
In order to resolve the above problem, in the preparation process of the catalyst for producing alkenyl acetates according to the present invention, the conventional reducing process utilizing liquid reducing agents in liquid phase is changed into a reducing process utilizing gaseous reducing agents in gaseous phase. As a result, the catalyst comprising palladium and a metal as catalysis promoter in combination with an alkali and alkaline earth metal compound supported on a carrier prepared by using this present invention has a high metal surface area and exhibits high catalytic activity, thus the catalytic efficiency and life of this heterogeneous catalyst are improved.